1. Field of Invention
This invention relates to systems and methods for processing image data prior to compression.
2. Description of Related Art
Data compression is often desirable for speedy data transmission. The compression incorporated normally exploits redundancy in the data to encode represent the input data with fewer bits. In imaging applications, an additional degree of freedom can be utilized as the final use of the data is for the visual inspection of a human. The transmitted data thus has to be visually correct, rather than bitwise correct with the input data. This is commonly used in image compression methods. Additionally, it has to be appreciated that the actual display size of an image has a strong interaction with the perceived contrast of the image by a human observer and that at different size levels, different contrast levels are preferred or desired. For example, a common image format is graphics interchange format (GIF). The graphics interchange format is a color mapped 8-bit/pixel format that embeds a predictive lossless coding as part of the file format.
A basic feature of the predictive lossless coding is that future data is predicted based on past data and that the result of the prediction is an intermediate data that will be compressed. Here, advantage is taken from the fact that good predictions, e.g., for highly correlated data, will often cause a zero prediction error. This leads to large sections of zeros in the intermediate data and thus to a better compression. It should be noted that we use “zeros” and “correlated” interchangeably in the subsequent sections.
Regardless of its quality and advantages/disadvantages, the graphics interchange format is the most common browser supported image file format. It is the format of choice for many devices for color images, since it can be understood by virtually all clients without any additional software installed. Here and in the future we will use GIF to represent all data formats that incorporate a reduced color set and lossless or quasi-lossless compression and are supported by common browsing clients.